5700 23RD DR W 2016-01-01 MF Import

� ��n�' �O�m' �0��d beYo� the boundaries of the subject site to the north an�i west of the P�Posed wesurn detentian pond consists of residendal developments. Sediments underlying the area of the proposed easurn detendon pond, located north of Loc 3_ E. ge�rally consist of glacially consolidaud, gravelly, silty sand (Vashon till). glacially consolidated sanci with minor amounts of silt and grav�( (V�bon advance outwash), and B��IIy consotidated silt (pre-Vashon undifferentiaced sediments). As was found in th� are� of the western detention pond, glacially coasolidaud sediments in the area of the eastern detendon pond were found to be overlain in places by several feet of loose colluvial sedimenu. The topogrdphy of che area proposed for the easum detendon pond generally slopes down towazd the notth at a gradient of approximately 5 to 10 percent, East of the proposed locadon of che eastem deuntion pond, the topography slopes steeply downward toward Narbeck Cmk to the east. The area north of the proposed eastern detendon pond locadon, outside of the 1�oundarizs of the subjecc si[e, consists of a residential development. RECOMMENDED BUILDING SETBACKS TO minimi�r �� sk of damage and en�i��n� � acts from building setback limit of 50 feet from the top of slope above Narb�k �Kg �ru p��Om the too of slope above East Fork Powder Mill Cmk. ; STOriM WATER DETENTION PO1VD CONSIDERpT10NS . The ��ra1, glacially consolidated sediments encouncered at the proposed locaaon of the two storm water detendon ponds aro suitable for the detention pond construcdon. The Vashon advance out�,ash S� �derlyirn rhe area of the prop�� pon ponds is relatively ��0� �use infiltradon of storm waur into flu Soils u�r�y�g �e deundon pond could potentially impact the reside�es (ocated downsldpe co the north an� W,est, azid aiso pountially destabiliu the steep slopes above East Fork Powder MiU Crcek and Nazb�k Creek, we recommend that both of the proposed ponds be I� �� PVC liners with a 30 mi1 minimam Uuclmess. The pond liners should be overlain by a minimum of one foot of structurai fill compacted to at least 95 percent of the modified proctpr �m� density (ASTM:D-1557). The structural fill should consist of nladvely imperauabk maurial such as the on-site till soils. AII penetrat�ons through pond liners must be sealed, For construcdon of li�d detention ponds we recommend a mini�� s��ck of 25 feet from the top of slope above Easc Fork Powder Mill Creek and a m�� yf 50 feet from the top of slope above Nazbeck Creek. I� maY be feasible to construct both detendon ponds without Iiners provided the ponds are �� back sufficiendy from the adjacent proporties to the north and fr�m �� tops of the slopes above East Fork Powder Mil] Creek and Narbeck Creek. Determinauon of suitable secbacks i would require modeling the grou� Water mounding effects and flow direcdons that would 2